In the field of silicon semiconductor device manufacturing, noble metal silicides, e.g., the silicides of titanium (Ti) and cobalt (Co), have been used to form contacts to a semiconductor region where a diffusion or an implantation has been previously done, and to polysilicon to provide low contact resistance or highly conductive interconnects. Titanium and cobalt are preferred because they offer low contact resistance with silicon. The electrical resistance of titanium silicide and of cobalt silicide are the lowest among noble- and refractory-metal silicides. Moreover, their silicides can be formed on silicon in a self-aligned manner. That is, these silicides can be formed at locations having predetermined chemical properties, thus rendering the use of an additional mask superfluous. Moreover, titanium silicide will be formed upon deposition of titanium on silicon even in the presence of an oxide layer over the silicon up to a thickness of approximately 40 angstroms, thereby avoiding the need for a separate oxide removal step, e.g. sputter-etch.
The refractory metals such as tungsten (W) and molybdenum (Mo) have received much attention as well because of their low electrical resistivities and favorable characteristics with regard to use as contact opening or via opening fills. The use of chemical vapor deposition (CVD) of tungsten for fabricating interconnections provides good step coverage. Also, tungsten and molybdenum, selectively deposited through the CVD process, can be used as a diffusion barrier, as a polysilicon shunt to reduce interconnection resistance, and as an interconnection in multi-level metallization.
In R. C. Ellwanger et al., Tungsten and Other Refractory Metals for VLSI Applications II (Mat. Res. Soc. 1987), Procs. 1986 Tungsten Workshop, Palo Alto, Calif., 12-14 Nov., 1986, pp. 385-394, it has been reported that selective growth of tungsten on top of monocrystalline silicon produces tunnel defect structures. Attempts to alleviate the potential reliability problems posed by such defect structures include growing tungsten onto regions where a silicide has previously been formed. The silicide enables low-resistance contacts to be achieved.
It would be highly desirable to merge the formation of titanium silicide or cobalt silicide with the CVD-tungsten step into a single process for reaping the fruits of both. However, such a merging would encounter a number of difficulties. Since CVD-tungsten involves fluorine chemistry, the vapor deposited substance may interact with the underlying silicide, thereby degrading the electrical properties such as contact conductivity and reliability.
In E. K. Broadbent. et al., J. Electrochem. Soc., 133, 1986, p. 1715, it is reported that titanium silicide is incompatible with CVD-tungsten using the source gas tungsten hexafluoride. The high reactivity of fluorine and titanium gives rise to the production of a highly resistive layer of non-volatile titanium tetrafluoride and therefore to high contact resistance. Alternatively, CVD-tungsten applied on cobalt silicide proceeds via the reduction of tungsten hexafluoride by consuming silicon from the silicide. In P. van der Putte et al., Appl. Phys. Lett. 49 (25), 22 Dec. 1986, pp. 1723-1725, the formation of local intrusions in the cobalt silicide layer is reported as a result of the CVD-tungsten, indicating encroachment of tungsten hexafluoride down the edge of the silicon-silicide interface. These and related silicide, as regards the undesirable susceptibility to the vapor chemistry associated with CVD processes, would therefore render such merging impractical.
In Ellwanger (op. cit.) a blanket CVD-tungsten process is reported, a major aspect whereof relates to the adhesion of CVD-tungsten to silicon oxide. The effect upon contact resistance to titanium silicide of this tungsten deposition process in combination with several adhesion layers to oxide is described. One of the combinations mentioned includes a tungsten silicide adhesion layer between titanium silicide and a CVD-tungsten blanket. This tungsten silicide layer is formed by means of a CVD process using a mixture of silane (SiH.sub.4), tungsten hexafluoride and hydrogen, the silane being provided for growth rate enhancement. It was reported that the silane chemistry is effective in reducing fluorine incorporation in the underlying titanium layer. However, this blanket deposition approach appeared to be rather unsuccessful in that it did not provide adhesion at the one-micron diameter contact level.